REPRESENTATIVE DESCENDING TRACTS

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1 REPRESENTATIVE DESCENDING TRACTSGeorgia Bishop PhDDepartment of NeuroscienceThe Ohio State University College of MedicineWe’ll now complete our discussion of the brainstem as a conduit by looking at some descending tracts.

2 At the end of this module you will learn to:OBJECTIVESAt the end of this module you will learn to:Trace the descending fiber tracts listed and discussed in the e-learning module. For each tract, you should be able to:Identify the location of the neurons that give rise to the tract.Define the primary motor function of the descending tractsName the location and immediate anatomical relationships of each tract in different subdivisions of the brainstem and forebrainDescribe the course of each tract from its origin to its ultimate site of termination including the location of any second and third order relays, whether it is ipsilateral or contralateral, and, if contralateral, identify the location where axons cross the midline (decussation)Be able to identify fiber tracts at all levels of the brainstem on cross sectional images from anatlas and on MRI/CT scans.Describe the location and relationships of relevant descending tracts in the CNS on gross brain specimens, cross sectional material, and radiographic images.

3 REPRESENTATIVE DESCENDING TRACTSMotor neurons in the spinal cord are capable of activating muscles through various reflex loops (e.g., Extensor reflex, flexor withdrawal reflex). The function of descending tracts is to regulate the activity of motor neurons in the spinal cord.**Lateral CorticospinalVentral (Anterior) CorticospinalRubrospinalMedial Longitudinal Fasciculus Including Medial VestibulospinalLateral VestibulospinalLateral ReticulospinalMedial ReticulospinalRaphespinalHypothalamospinal****CORTICOSPINAL – Classically Defined As Mediating Voluntary MovementRUBROSPINAL – Alternate Route For Mediation/Modulation Of MovementsMLF – 1) Input From Vestibular System For Postural Adjustments and Coordination of Head and Eye Movements.2) Input From Nuclei Controlling Mucles of Eyes For Coordination Of Eye Movements (III, IV, VI)Motor neurons in the ventral horn of the spinal cord are capable on their own of stimulating muscles to contract through a variety of reflex loops. Think about having your knee tapped and the rapid extension of the leg. This does not require any cortical input. It is all governed at the level of the spinal cord. So what is the function of descending pathways? Basically, the regulate the activity of these motor neurons. In the absence of descending input from the cerebral cortex or other parts of the brainstem the spinal cord would just keep responding to every small stimulation and make the muscles contract. A perfect example of this is a stroke patient or a spinal cord injured patient that has lost descending control of their motor neurons. Often, these individuals are in a state of spasticity where muscles on both sides of the joint are contracting at will. There *** are many descending pathways that control motor neurons as shown here. As for the ascending pathways, we won’t go through all of them now. Three *** will be reviewed in a bit of detail. Others will be discussed during the motor portion of this block. The corticospinal tract *** is classically defined as mediating voluntary movement. The rubrospinal tract is an alternate route for mediating or modulating movements. It is influenced primarily by the cerebellum which is essential for coordination of movement. The medial longitudinal fasciculus was mentioned in the module on ascending pathways and is a pathway for coordination of eye and head movements and involves vestibular input as well as input from motor nuclei that control eye movements. Let’s look at each of these pathways.

4 MOTOR CONTROL COMPONENTSLOWER MOTOR NEURONSMotor neurons in spinal cord and cranial nerve nuclei that project to skeletal muscles.Final common pathway to innervate muscles. When lost – flaccid paralysisNEURON IN BRAINUPPER MOTOR NEURONSNeurons in cortex and other areas of brainstem that project to motor neurons in spinal cord or cranial nerve nuclei with somatic motor component.Damage results in spastic paralysis as reflexes go “unchecked”.Let’s first define a few terms. One is ***Lower Motor Neuron. A lower motor neuron is the neuron that actually innvervates skeletal muscle. This may be an alpha motor neuron within the spinal cord or a motor neuron in one of the cranial nerve nuclei that innvervates skeletal muscle in the face (e.g., the hypoglossal nucleus contains lower motor neurons that innervate the tongue). If these neurons are lost, the muscle become flaccid. ***Upper Motor neurons are neurons located in the cerebral cortex or other brainstem centers that synapse on lower motor neurons and control their firing rate. Loss of these neurons results in a spastic paralysis in which muscles are hypersensitive to sensory input and are in almost constant state of contraction. Several *** other areas of the brain influcence the activity of upper motor neurons including the basal ganglia, the cerebellum, and other areas of the cerebral cortex. We will discuss all of these in due time.Basal ganglia, cerebellum, association areas of motor cortex play vital role in the choice, design and monitoring of movement but have no direct effect on lower motor neurons.

5 CORTICOSPINAL (PYRAMIDAL) TRACTPRECENTRALGYRUSCELLS OF ORIGIN: Pyramidal cells in motor regions of frontal lobe.CEREBRALCORTEXINTERNALCAPSULEAxons course through posterior limb internal capsule (large fiber tract located lateral to thalamus).Axons descend ipsilaterally through cerebral peduncle, basilar pons and rostral pyramidal tract in the medulla.CEREBRALPEDUNCLEMIDBRAINOn reaching caudal medulla 85% cross to the contralateral side in the pyramidal decussation to form lateral corticospinal tract.PONS15% remain ipsilateral to form anterior corticospinal tract.As we did for the descending systems, we will summarize the tract in a single slide. As before, be sure you know what is on this slide. Let's start with the corticospinal tract. As it's name implies, this tract begins in the cerebral cortex,*** primarily in regions in the frontal lobe that control motor activity. The axons ***course through a large fiber tract in the subcortical white matter called the internal capsule. This tract is located just lateral to the thalamus. ***The axons continue to descend ipsilaterally through the cerebral peduncle and into the basilar pons. On leaving the pons, ***they form the pyramidal tract in the medulla. Approximately 85% of the axons in the pyramidal tract cross the midline in the caudal medulla and form a tract in the lateral funiculus of the spinal cord called the lateral corticospinal tract. The *** 15% of axons that remain ipsilateral form a tract called the anterior corticospinal tract which remains in the ventral funiculus; basically it continues the course of the pyramid. The axons in the lateral and anterior corticospinal tract terminate*** directly on motor neurons or on interneurons in the spinal cord that then contact motor neurons. Unlike the sensory system, this is a tract with a single synapse. Cortical neuron to spinal cord neuron to muscle.MEDULLAPYRAMIDPYRAMIDALDECUSSATIONTerminate either directly on motor neurons or on interneurons that contact motor neurons.C8LATERALCORTICOSPINALTRACTL4TO SKELETAL MUSCLE

6 CORTICOBULBAR TRACTNeurons in the cerebral cortex also project to motor nuclei related to cranial nerves. Arise from face region of precentral gyrus.Although shown as bilateral input, V, X, XI and XII primarily receive crossed projection.ICCPINPUT TO VII IS COMPLEX.Cortical input is primarily crossed to innervate muscles of upper and lower face .Upper facial muscles receive a bilateral input from the cortex.These muscles may be spared when there is a stroke on 1 side of the cortex. This is important for distinguishing between cortical and peripheral injury to facial nucleus/nerveMotorTrigeminal(V)Facial MotorNucleusContra: Upper FaceAnd Lower FaceUNABLE TO BARE TEETH ON LEFTSYMMETRIC ELEVATION OF FOREHEADFacial MotorNucleusIpsi: Upper FaceThe tract we just described accounts for control of spinal motor neurons. What about those to muscles in the face. We know there are several cranial nerve nuclei that have somatic motor components that require cortical control. Axons in motor regions of the cerebral cortex, in particular in regions related to the face, leave the corticospinal tract in the midbrain, pons, and medulla and project to cranial nerve nuclei that control movement of muscles in the head. These axons are called corticobulbar axons. For the most part the input is primarily bilateral with a stronger contralateral input; just as we saw with the corticospinal tract. It is worth noting that the input to the motor nucleus of the facial nerve from the cortex is a bit more complicated. ***As for other cranial nerve nuclei, the input is primarily crossed as it innervates neurons that project to muscles in the upper and lower parts of the face. However, the upper facial muscles actually get a strong bilateral input from both ipsilateral and contralateral upper motor neurons in the cortex. *** If there is a stroke involving one side of the cortex, the patient may still be able to contract muscles in their forehead as shown in the image. If a facial weakness is thought to be due to damage at the level of the facial nucleus or the facial nerve, all movement on one side would be lost. This allows you, as a physician, to differentiate between a stroke or facial nerve damage in a patient with weakness in their muscles of facial expression.HypoglossalNucleus (Xii)Nucleus AmbiguusMuscles Of Larynx,Pharynx (IX, X)Accessory MotorNucleus (Xi)

7 Descending Fibers Include: Corticospinal INTERNAL CAPSULEALL AFFERENTS AND EFFERENTS RELATED TO THE CORTEX TRAVERSE THE INTERNAL CAPSULE.Internal Capsule is a large fiber bundle located lateral to the thalamus and the caudate nuclues.ICINTERNAL CAPSULE:Descending Fibers Include: CorticospinalCorticobulbar Axons CorticopontineInternal Capsule:Ascending Fibers Include:Medial LemniscusSpinothalamic TractLet's digress for a moment to define a structure described on a previous slide called the internal capsule. This is a large fiber tract through which all afferents and efferents to and from the cerebral cortex travel. It is located lateral to the thalamus and medial to a collection of subcortical nuclei called the basal ganglia. Since we are talking about descending axons here, let's just consider which ones are present in the internal capsule. *** These include the corticospinal and corticobulbar axons we just described as well as axons that arise in the cortex and terminate in the pons; these are related to the cerebellum. Ascending axons*** include the two tracts we talked about in the last e-learning module, namely the medial lemniscus and the spinothalamic tract.

8 CORTICOSPINAL TRACT AND INTERNAL CAPSULEMPLINTERNALCAPSULE:ALTHPUTICMRI – PATIENT WITH DISEASE (ALS) LEADINGTO DEGENERATION OF UPPERMOTOR NEURONSGENULocation OfCorticospinal/CorticobulbarTract In PosteriorLimb Of InternalCapsulePLThe corticospinal, and corticobulbar tracts occupy a very specific portion of the internal capsule. This is a horizontal section through the dorsal part of the diencephalon. The red circle *** indicates the location of the corticospinal tract in a portion called the posterior limb. How do we know this. Tract tracing experiments in animal models have confirmed it. Also, there is data from humans. This *** is an MRI from an individual suffering from Amyotropic Lateral Sclerosis or ALS or Lou Gehrig’s disease. In this disease, motor neurons are selectively destroyed for as yet unknown reasons. The white patches indicated by the red arrow represent areas where motor axons have been lost. As predicted from experimental studies, they are in a very specific region of the internal capsule.THALAMUS

9 CORTICOSPINAL TRACT - MIDBRAINIn The Midbrain, The CST and CBT Occupy the Middle Part Of The Cerebral Pecuncle (CP)CPMRI – PATIENT WITH DEGENERATION OF UPPERMOTOR NEURONSSNTECTUMTEGMENTUMCPSCSTTMLRNThe internal capsule continues caudally as the cerebral peduncle. Let's continue following the corticospinal tract from the diencephalon to the spinal cord. This is a section through the midbrain. The corticospinal and corticobulbar tracts occupies the middle part of the cerebral peduncle in the area indicated by the red box. How do we know this? ***Here is an MRI from the same patient. The area showing loss of axons arising from upper motor neurons is indicated by the red arrows. The white patch shows they were lost from the center of the cerebral peduncle.

10 CORTICOSPINAL TRACT - PONSIn the pons the CST is broken up into fascicles that course between collections of neurons forming pontine nucleiMany cortical axons, not contributing to the corticospinal tract, project to brainstem nuclei in the midbrain, pons and medulla, especially cranial nerve nuclei with a motor component. In addition axons, terminate on neruons in the reticular formation.IVPNCSTMLSTTMLFSCPIn the pons, the corticospinal and corticobulbar tracts get broken up into many fascicles by the pontine nuclei. These axons have a cross cut appearance in transverse sections since they are running perpendicular to the plane of the section. Many axons leave these bundles to innervate nuclei in the pons such as the motor nucleus of V and the facial motor nucleus. Other axons leave the bundle to innervate neurons in the pontine nuclei, located in the clear areas between myelinated axons. For reference, the medial lemniscus and spinothalamic tract are labeled for your reviewing pleasure.

11 CORTICOSPINAL TRACT – MIDBRAIN VS. MEDULLAIn the medulla, the fibers regroup to form the pyramidal tract (pyramid). However, there is a significant decrease in size of this fiber tract compared to that of the cerebral peduncle.CPPYThese sections from the midbrain and medulla are shown at the same magnification. Focus on the cerebral peduncle and the pyramids. This gives you an idea of how many axons actually leave this large fiber bundle within the pons and upper medulla. The cerebral peduncle is 4-5 times the size of the pyramidal tract. Once you reach the pyramidal tract, the only axons remaining are those destined for the spinal cord. This is where you can clearly define the corticospinal tract itself.COMPARISON OF SIZE OF CEREBRAL PEDUNCLE (BEFORE PONS) AND PYRAMID (AFTER PONS) TO SHOW HOW MANY AXONS LEAVE THE TRACT IN THE PONS AND ROSTRAL MEDULLA. THE IMAGES ARE AT THE SAME MAGNIFICATION.

12 CORTICOSPINAL TRACT - DECUSSATIONAt The Junction Of The Spinal Cord And Medulla, 85% of the Axons in the Corticospinal Tract Cross to the Contralateral Side. Note, They Also Move Dorsally And Laterally. This is Now Called the Lateral Corticospinal Tract.15% Of The Fibers Remain on the Ipsilatral Side in the Same Ventral Position. This is Now Called the Anterior Corticospinal Tract.PYRAMIDAL DECUSSATIONPYPYAt the junction of the spinal cord and the medulla the clear line of separation between the right and left pyramid becomes obscured as indicated by the dashed line. This is the point where the corticospinal axons begin to decussate or cross the midline. Note *** in this cross section of the spinomedullary junction, that the axons do not cross directly from right to left across the midline. Rather, they move from ventral to *** a position that carries them dorsal and lateral. As we’ll see, this will correspond in space to the location of the lateral corticospinal tract in the lateral funiculus of the spinal cord. The few fibers that do not cross, remain on the ventral surface of the brainstem into the spinal cord. These are indicated by the X *** on the gross brainstem.DXXPY = PYRAMID; CORTICOSPINAL TRACTDX – PYRAMIDAL DECUSSATION

13 CORTICOSPINAL TRACT – Spinal CordLATERAL CST AXONS TERMINATE ON:Motor Neurons Located Laterally (Upper Limb And Digits)Interneurons That Project To Motor NeuronsNeurons At Base Dorsal HornCERVICAL CORDLCSTANTERIOR CST AXONS TERMINATE ON:Motor Neurons Located Medially (Axial Muscles).Some Cross To Contralateral SideEnd In Cervical And Thoracic SegmentsaCSTaCSTTHORACIC CORDLCSTPYRAMIDALDECUSSATIONThese are 2 sections through the cervical and thoracic spinal cord. *** Lateral corticospinal axons are located in the lateral funiculus which is where the ended up after the decussation. Remember, axons the right originated in the left cerebral cortex and vice versa. The few uncrossed axons remaining *** on the ventral surface are the anterior corticospinal tract. They are in the same relative position as the pyramids. The axons in the lateral and anterior corticospinal tract terminate either directly on motor neurons in the ventral horn or on interneurons. Thos in the lateral corticospinal tract will synapse primarily on motor neurons that innervate distal muscles such as those that innervate digital or wrist muscles. Those in the anterior corticospinal tract primarily synapse on motor neurons that innervate trunk muscles located more medially.aCSTLCSTC8aCSTL4

14 RUBROSPINAL TRACT CELLS OF ORIGIN: RED NUCLEUS IN THE MIDBRAINReceives input from cerebral cortex and cerebellum and modulates activity of motor neurons in the spinal cord..REDNUCLEUSAxons cross midline to form the rubrospinal tract (RST) in midbrain and descend in lateral tegmentumMost terminate in cervical spinal cord.MIDBRAINRST (CTT)In spinal cord, axons are located just ventral to lateral corticospinal tractPONSLet’s now turn our attention to another descending tract called the rubrospinal tract. In this diagram, both the corticospinal and the rubrospinal tracts are shown. The cells of origin of the rubrospinal tract are located in the red nucleus. Remember, this is a nucleus that is located in the midbrain. The red nucleus receives input from both the cerebellum and the motor parts of the cerebral cortex. It’s function is to modulate the activity of motor neurons in the spinal cord, in particular flexor muscles related to proximal muscles. The axons arising from the neurons in the red nucleus cross the midline*** as they exit the nucleus, that is in the midbrain. The axons descend through the tegmentum of the pons and into the medulla where the tract is located just dorsal to the inferior olivary nucleus. In the spinal cord *** the axons are located immediately ventral to the lateral corticospinal tract. Compare the trajectory of the rubrospinal tract to the corticospinal tract. The corticospinal tract remains ipsilateral until the axons reach the caudal medulla. Rubrospinal axons cross the midline at their point of origin in the midbrain. Corticospinal axons are essential for movement of the digits and the hand whereas the rubrospinal tract is primarily concerned with controling proximal limb muscles.ROSTRALMEDULLACERVICALSPINALCORD

15 RUBROSPINAL TRACT RST TEGWe will now follow the rubrospinal tract through a series of cross sections beginning at the level of the midbrain. The red dot represents a neuron within the red nucleus. On emerging from the nucleus, the axon crosses the midline and enters the tegmentum of the pons. As the basilar pons diminishes just rostral to the medulla the rubrospinal tract assumes a location just dorsal to the inerior olivary nucleus. We’ll pick it up in the spinal cord on the next slide.RSTRSTTOSPINALCORD

16 RUBROSPINAL TRACTLCSTIn the spinal cord the rubrospinal tract is located adjacent to LCSTRSTCERVICALLCSTThese sections are through the cervical and thoracic spinal cord. The lateral corticospinal tract is outlined in yellow at both levels. The rubrospinal tract *** is located immediately ventral to the corticospinal tract at both cervical and thoracic levels. In lower mammals, the rubrospinal tract is actually larger and more important than the corticospinal tract. It is only in humans and in apes that have fine control of their digits that the corticospinal tract enlarges and takes on a more prominent role.LCSTRSTTHORACIC

17 MEDIAL LONGITUDINAL FASCICULUSFiber Tract Extending from the Midbrain to the Spinal Cord that Connects the Three Extraoculuar Nuclei that are Responsible for Coordinated Movements of the Eye: Oculomotor, Trochlear And Abducens.Also Contains Input from the Vestibular Nuclei to Coordinate Movement of the Head and Eyes.MLFTO CERVICALSPINAL CORDMIDBRAINOCULOMOTORNUCLEUS(III)TROCHLEARNUCLEUS(IV)ABDUCENSNUCLEUS(VI)The final descending tract we will discuss is the medial longitudinal fasciculus or the MLF. The MLF is actually both an ascending and a descending fiber tract extending from the midbrain*** to cervical levels of the spinal cord. This tract interconnects the vestibular nuclei and the 3 cranial nerve nuclei that control movement of the eyes, namely the oculomotor, trochlear, and abducens. It's function is to control eye movements during movements of the head. Think about turning your head to the right, the normal movement of the eyes is to the left to maintain focus on an object. This is not as simple as it seems. To move your eyes to the left requires activation of the left abducens nucleus and the right oculomotor nucleus (activating the lateral rectus on the left and the medial rectus on the right). The movement of the head is detected by specialized organs in the inner ear. You will learn more about the vestibular nucleus later in the block. For now, let's just look at the pathway. Head movement activates neurons within the vestibular nuclei. ***Their axons course medially to begin forming the MLF. Some axons descend to the cervical spinal cord to control motor neurons that innervate neck muscles for head movement. Ascending vestibular axons leave the MLF and synapse on neurons in theabducens nucleus***. Axons from the abducens nucleus course medially to join the MLF. These axons course rostrally and cross the midline, not shown here, where they innervate the trochlear nucleus *** and the contralateral oculomotor nucleus ***. Axons from the oculomotor nucleus also join the MLF to provide feedback to the abducens nucleus. As you can see, this is basically a network to organize eye movements.VESTIBULARNUCLEUS(VIII)

18 MEDIAL LONGITUDINAL FASCICULUS - VESTIBULAR INPUTVNVIIVIIIMLF – ASCENDING LIMBMLF – DESCENDING LIMBAscending and descending limb of the MLF begin in the vestibular nucleiAscending limb links vestibular nuclei and nuclei that control movements of the eye (III, IV, VI)Descending limb links vestibular nuclei with interneurons and motor neurons in the cervical spinal cord.This is another view of the MLF on a diagram of the brainstem. As described before neurons in the vestibular nucleus project to the abducens nucleus which is located adjacent to the vestibular nuclei. You can see how the MLF links the vestibular nuclei with the abducens, trochlear and oculomotor nucleus. The descending axons arising from the vestibular nucleus descend to the cervical spinal cord to control neck muscles.

19 DESCENDING LIMB MLF ARISES FROM THE VESTIBULAR NUCLEI AND CONTROLS NECK MUSCLESMEDIALVESTIBULOSPINALTRACTMedial VestibulospinalTract = descending MLFDescending part of MLF (a.k.a. Medial Vestibulospinal Tract) is bilateral. Most axons stay ipsilateral but many cross. Remain immediately adjacent to midline in medulla in tract located immediately dorsal to medial lemniscusHere is one last view of the descending limb of the MLF. It is also called the medial vestibulospinal tract. As it descends some of the axons cross the midline and others remain ipsilateral. In the spinal cord, the axons are located within the ventral funiculus. The location of these axons places them in a position to innervate neck muscles.Axons enter ventral funiculus of spinal cord and terminate primarily on motor neurons that innervate axial musculature of neck.Medial Vestibulospinal Tract is not found below cervical levels.

20 MLF IN THE MEDULLA AND PONSIN THE PONS, THE MLF RETAINS ITS POSITION DORSALLY AND MEDIALLY AS THE MEDIAL LEMNISCUS RE-ORIENTS FROM VERTICAL TO HORIZONTAL AS THE BASILAR PONS EXPANDS.MLVIMLFVNMLF (MVST)MLSTSTLet’s now follow the MLF through the brainstem. The MLF begins at the level of the vestibular nuclei in the rostral medulla.*** At this point it is located immediately dorsal to the medial lemnisus. ***In the pons, the MLF retains it’s position on the dorsal, medial aspect of the pontine tegmentum, even as the medial lemniscus shifts to a horizontal orientation. For review, the spinothalamic tract is also labeled in this section. *** The dorsal region is shown at higher magnification in this picture. The MLF is outlined in yellow. At this level, axons derived from neurons in the abducens nucleus *** are added to the MLF. Likewise, axons from the vestibular nuclei ***leave the MLF to innervate neurons in the abducens nucleus.VI

21 MLF IN THE ROSTRAL PONS IVMLF (MVST)MLSTDSCPIn the rostral pons, the MLF retains its midline position. At this level, axons from the trochlear nerve are added to the MLF.IV

22 MLF IN THE MIDBRAINMLSTRNCCSNMLFIIIIVIIIIn the midbrain, again the MLF is in the same midline position in the dorsal part of the tegmentum. At this level, axons from the oculomotor nerve join the MLF. Also, at this level, ascending axons leave the MLF to innervate oculomotor neurons.

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